The present invention relates generally to a system and a method of controlling a vehicle equipped with a continuously variable transmission, and more particularly to estimating an input torque to a continuously variable transmission.
Vehicles usually have an internal combustion engine coupled to an automatic transmission, which could be a Continuously Variable Transmission (CVT). In a CVT, which typically comprises a metal belt sliding on two pulleys (a primary pulley coupled to the engine and a secondary pulley coupled to the drive shaft), the gear ratio can be continuously changed by applying hydraulic pressures to the pulleys and thus changing the belt winding radius. By continuously varying the transmission ratio, the powertrain of the vehicle equipped with CVT can be controlled such that actual engine speed is matched to the desired engine speed at which most efficient fuel consumption can be achieved. Thus, a CVT equipped vehicle can achieve improved fuel efficiency throughout the entire range of vehicle speeds made available by the infinitely variable driveline as compared to vehicles with the conventional automatic transmission.
The inventors herein have recognized that a CVT system may require a controller for determining a proper amount of clamping forces to be applied to the primary and secondary pulleys to prevent the belt from slipping. The clamping force to the secondary pulley is determined based on the amount of estimated CVT input torque, and the clamping force to the primary pulley is adjusted such that desired transmission ratio is achieved. The inventors herein have further recognized that the accuracy of the CVT input torque estimate is important to the proper operation of the Continuously Variable Transmission. If the torque estimate is too low, the clamping force will also be too low, and the belt can slip, which may result in rapid degradation and wear of the belt. Additionally, the CVT requires much higher clamping forces than the conventional transmission (typically 2-4 times higher) to prevent the belt from slipping on the pulleys. Therefore, if the clamping force estimate is too high due to inaccuracies in the CVT input torque estimate, the fuel economy penalty could be very significant. In other words, over- or underestimating the CVT input torque could result in reduced vehicle durability, fuel economy and overall customer satisfaction.
The CVT input torque is estimated based on the information provided by sensors such as the engine speed sensor, mass airflow sensor, air-fuel ratio sensor, etc. The inventors have further recognized that it may be possible for these sensor signals to drift in range due to electrical degradation or chemical contamination. Therefore, since the CVT input torque estimate may not be accurate due to sensor as well as engine tolerances, the resulting secondary pulley clamping force estimate which is based on the CVT input torque estimate may also not be as accurate as possible.
The inventors have additionally recognized that an accurate estimate of the CVT input torque could be obtained based on a torque converter ratio when the torque converter clutch is unlocked. The accuracy of this estimate is affected mainly by the variations of the torque converter ratio (which depend on the converter design tolerances), and may be more accurate than the engine torque-based estimate, which is affected by engine, sensor and torque converter tolerances.
The disadvantages of prior approaches are overcome by a method for controlling a powertrain of a vehicle having an engine coupled to a continuously variable transmission, the vehicle further having a torque converter coupled between the engine and the transmission, including: providing an indication that a torque converter clutch is unlocked; learning a powertrain torque correction value when said indication is provided; adjusting a powertrain torque estimate based on said correction value; and determining a control signal for adjusting a transmission clamping force based on said adjusted powertrain torque estimate.
In other words, this method determines correction coefficients when the torque converter clutch is unlocked. These coefficients are then used to calibrate an engine model such that the error between the more accurate torque converter-based CVT input torque estimate and the less accurate engine-based CVT input torque estimate is minimized. Thus, by controlling a transmission clamping force based on an adjusted powertrain torque estimate, it is possible to provide just enough force to transmit the torque, without providing too much force, which can waste fuel.
An advantage of the above aspect of invention is that improved fuel efficiency can be achieved by eliminating overestimating of clamping forces. Also, improved reliability and customer satisfaction will be achieved by preventing CVT belt slippage due to clamping force underestimation.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.